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'90 legacy automatic - transmission/diff noise??

'90 legacy wagon auto trans, 150,000 miles...
now hearing a pulsing grinding noise in the auto transmission or differentiial .. differential was out of oil ..no sign of leakage .. refilled with oil ..still makes grinding noise ... noise occurs when driving not idling .. while driving about 30 mph when noise is audible ..put in neutral and noise stops ..but then the noise returns, if you rev up the engine while still in neutral while coasting the noise comes back ...any thoughts??? much appreciated

I sounds like it could be torque bind, or another problem with the driveline. Given the age and mileage on your car it is likely that you have a worn out torque transfer assembly (the AWD system).

I am not sure why the noise would disappear when you shift into neutral, and then return when you rev the engine. Perhaps the duty solenoid has failed and when you shift to neutral the clutch pack release a bit, and revving the engine increases hydraulic pressure in the transmission causing the clutch pack to bind again.

Try the following before doing any work:

1. If all four tires are NOT the same model and size, install four identical tires.

4. Check ATF level (dipstick) note the hot and cold marks, and smell fluid to see if burnt. If burnt you need a minimum of new fluid, and perhaps transmission work.

5. check axle shafts (all four) for torn CV boots. If a boot is torn and has been driven in that condition for a while the CV joint may be bad. A joint may still be bad even with a good boot. Do you get any clacking from an axle when accelerating through a turn? If so you have a bad CV, if not your CV's are most likely ok.

6. You could have a worn wheel bearing, the rears are prone to failure. Is the vibration in your seat or steering wheel? If steering wheel, it is likely a front bearing. If noise is more pronounced on left turns, it is likely the right bearing and vice versa. If the vibration is from the seat it is likely a rear wheel bearing. Someone will need to sit in back to listen to which side it is from, it can be difficult to hear which side from the front seats.

7. Drive car in a straight line at slow speed (15MPH) and then turn sharply. Does the noise occur during the turn? If so you have torque bind, or a worn rear differential, or a problem with the driveshaft.

8. Measure the tire circumference. Subaru claims that all four tires must fall within a 1/4 inch range otherwise AWD system failure will result. In reality the AWD system is not that sensitive, but the closer the tires are in circumference the better. If you have greater than 1 inch between the smallest and largest tire, you may have a tire problem.

Unless wheel bearings or tires:

You should check the driveshaft for worn U joints, do this by grabbing either side of the U joint and trying to twist in opposite directions. There should be no play whatsoever.

Check the driveshaft hanger bearing, verifying that it is properly bolted to the underside of the car (I have seen these loose), and that it has no play (pry against the driveshaft with a pry bar).

The clutch pack discs will develop a rough surface over time that caused torque bind. This can be corrected by driving in tight circles at low speed in a parking lot. Doing 25 right circles followed by 25 left circles can sometimes help. Some people suggest up to 100 turns, but it shouldn't be nessessary. If 50 turns does not correct the problem, you have a more serious problem that requires removal of the tailshaft.

Given that your diff had no fluid (and I assume you mean rear diff), it is possible that you destroyed your diff. It will have to be checked for play, which is not easy to do. You need three hands to check free play, and some free play is normal:

Lock the input shaft, and check play between right and left output shafts by locking one side and turning the other.

Lock the output shafts and check play on the input shaft by turning it.

Used rear differentials are easy to come by, just be careful that the ratio matches your car. Also be aware the the viscous locking diffs have different output shafts, do if you have LSD get a replacment LSD, and if not get a regular diff. Diffs on Subaru's are very durable, unless they are run without oil. You can upgrade from a regular diff to LSD, but more parts are needed (axle shafts?)

If it is your front diff, it is not uncommon to have the fluid run low and not read on the dipstick. Drain the fluid and verify its condition. The drain plug is magnetic, are there any metal shavings on it? Some shavings on the magnet are normal. Clean the plug. refill the transmission with oil and drive for a day. Drain fluid again, are there new metal shavings? If so you may have a blown front diff (quite a rare problem). If your front diff is shot you may wish to install a whole used transaxle assembly, just be sure to get one that has a refurbished torque transfer assembly (post 1998 repair kit), or refurbish it yourself.

If it is not your tires, wheel bearings, clutch pack, rear diff, or front diff, you most likely have a failed torque transfer assembly. Either the duty solenoid has failed or the torque transfer seals are worn (most likely). This things always fail on high mileage pre 1998 Subaru's.

Most of the above checks that I noted can be done for little or no cost. I like to eliminate everything else before considering torque bind within the transmission, as I have seen conditions where something else causes torque bind.